This run-of-the-mill contest between two fairly conventional candidates could tell us what to expect in the midterm elections for the House of Representatives and the Senate.

That will determine whether Mr Trump remains able to push policy through Congress, where both the upper and lower house are currently controlled by the Republican party.

The Democrats will need to win 23 extra seats to take control of the House of Representatives and gain the ability to defeat his agenda.

Candidates in close races across the country are watching "canary in the coalmine" Ohio closely for clues as to what they should expect.

Ohio, vote today for Troy Balderson for Congress. His opponent, controlled by Nancy Pelosi, is weak on Crime, the Border, Military, Vets, your 2nd Amendment - and will end your Tax Cuts. Troy will be a great Congressman. #MAGA

Ohio's Republican governor and Trump critic John Kasich told ABC America suburban women in the state were "really turned off" by the "chaos" that surrounds the President. "It really doesn't bode well for the Republican Party because this shouldn't even be close," he said.

Mr Trump's attack on Ohio-born basketball star LeBron James has also not helped endear him to the state. The President won the state by eight points in 2016, after Barack Obama won it by three points in 2012 - but his star may be fading. In June, a Marist poll found 57 per cent of people in Ohio did not want him re-elected in 2020.

WHAT ELSE ARE WE WATCHING?

Four other states are holding standard primaries today - Michigan, Missouri, Washington and Kansas, and these could be significant, too.

Mr Trump has tweeted his support for secretary of state Kris Kobach in the race for Kansas governor.

This was against the advice of his aides, according to the Associated Press, who fear right-wing Mr Kobach could weaken the Republicans' hold on the state.

Alexandria Ocasio-Cortez, who pulled off an upset in New York in June, is supporting two progressive candidates in Kansas; a former Homeland Security official who would be the first Muslim woman elected to Congress in Michigan; and a pastor and civil right activist in Missouri.

These are nailbiting times for the GOP, with an unpopular President alienating many Republicans and galvanising the opposition.

Democrats are hoping a "blue wave" of anger at Mr Trump's divisive policies will send swing states left and erode his power before he wins another term.

With his eyes already on that prize, the commander-in-chief is - as ever - determined to succeed.